1. Field of the Invention
The present invention relates to a spin coating method and a spin coating apparatus that involves supplying a coating liquid to a substrate and forming a coating film thereon by rotating the substrate.
2. Background Art
The spin coating method is a method of forming a coating film by dispensing a coating liquid onto a substrate and then rotating the substrate at a number of revolutions for determining the thickness of the coating film.
The spin coating method is characterized in that the thickness of the film formed depends only on the number of revolutions and is highly uniform, and the process time is short. The spin coating method has a wide variety of applications, such as manufacture of semiconductor devices or liquid crystal displays.
A first representative application is patterning by lithography. A representative lithography process in manufacture of a semiconductor device involves spin-coating a substrate with a photoresist, drying the photoresist on a hot plate, exposing the photoresist to light, and performing post baking on the hot plate. Then, the photoresist is developed using a developer to form a pattern on the substrate.
In order to prevent reflection of light from the substrate in the exposure described above, an antireflection film is also formed by spin coating. There are two types of antireflection films. One is underlies the resist, and the other overlies the resist. In addition, in order to prevent volatilization of gas from the photoresist or provide protection against contaminants in the external environment, a protective film is also formed on the photoresist by spin coating. This is common practice in the immersion exposure process.
The resist process can be classified according to the layered structure of films. For example, if a photoresist film is formed by itself, it can be classified as a single-layer resist. If a photoresist film is formed on a thick base film (organic film), it can be classified as a double-layer resist. If a photoresist film is formed on a thin layer of an inorganic material stacked on a thick base film (organic film), it can be classified as a multilayer resist. The base film and the film of an inorganic material can also be formed by spin coating.
Furthermore, photoresists are classified as an organic photoresist or an inorganic photoresist according to the composition of the material. A commonly used inorganic photoresist is the silicon-based photoresist. The photoresist film of any material can be formed by spin coating. The photoresist film may be formed on a flat substrate or a stepped substrate.
In order to further reduce the size of fine patterns, a shrinkable material is used. For example, if a shrinkable material is applied on a resist pattern having a hole, and only the shrinkable material is removed using a developer after heat treatment, the diameter of the hole can be reduced. This is because the part of the resist that thermally reacts with the shrinkable material becomes insoluble in the developer.
A second application is formation of a polyimide film. Polyimide is a thermosetting resin. A polyimide film is formed by spin-coating a substrate with a polyimide solution, drying the polyimide solution on a hot plate and making the polyimide set by thermal treatment. The film is used as a passivation film, an interlayer insulating film, a stress buffer film or the like.
Furthermore, various kinds of patterns can be formed by lithography using a photosensitive polyimide.
A third application is formation of insulating films of spin on glass (SOG) or a low-k material, which has a low dielectric constant, for example. Any of these films is made of a material primarily containing silicon and is formed by spin coating and oxidation by thermal treatment or the like. The oxidized film is used as an interlayer insulating film, a pre-metal dielectric (PMD) film or an inter-metal dielectric (IMD) film or for shallow trench isolation, for example. The material of these insulating films may be applied to a flat substrate or a stepped substrate.
A fourth application is a formation of a thin film by a sol-gel method. The sol-gel method is a method of forming a metal oxide film, a dielectric film, a ferroelectric film, a conductive film or the like by sintering an organic solution containing a metal alkoxide in a gas phase containing water. That is, a thin film is formed by performing spin coating with a metal alkoxide solution, drying the metal alkoxide solution on a hot plate and sintering the metal alkoxide. This method is used for forming a ferroelectric film for a ferroelectric memory, for example.
Such a spin coating method is used in wide variety of applications other than those described above, such as application of an organic EL material, MEMS, and manufacture of an optical material.
A representative spin coating method involves dispensing a coating liquid to a central part of a surface of a substrate at rest and then rotating the substrate to uniformly spread the coating liquid over the entire substrate. However, this method requires a large amount of coating liquid for processing one substrate.
To overcome the disadvantage, a conventional spin coating method involves dispensing a coating liquid to a semiconductor substrate while rotating the semiconductor substrate at a low number of revolutions and then forming a coating film by rotating the substrate at a number of revolutions for determining the thickness of the coating film.
The conventional spin coating method has a problem that a considerable amount of coating liquid is required, and the peripheral part of the substrate cannot be coated when the substrate has a large area.
To overcome the disadvantage, another conventional spin coating method involves setting the number of revolutions in dispensing the coating liquid higher than the number of revolutions in determining the film thickness. This method can solve the problem to some extend and can be used for a large substrate (see Japanese Patent Laid-Open Nos. 8-330206 and 2004-64071, for example).
However, the spin coating method described above has a problem that a large amount of coating liquid is used.
By the way, if the wettability of the substrate to the coating liquid is poor, the coating liquid is repelled, so that a coating unevenness occurs, and the film thickness uniformity is degraded. To prevent the degradation of the film thickness uniformity, there is proposed a precoating method that wets the substrate with a solvent (flux) before coating (see Japanese Patent Laid-Open Nos. 5-123632 and 5-243140, for example).
The spin coating method is a method that involves spin-coating a surface of a substrate with a solvent before dispensing a coating liquid and spin-coating the surface of the substrate with the coating liquid (precoating method).
The precoating method requires a smaller amount of coating liquid and can make the film thickness uniform. Therefore, the precoating method is commonly used in cutting-edge photoresist processes.
The precoating method has a disadvantage that it requires an additional solvent supplying mechanism and a larger amount of solvent. In addition, in the case where a stepped substrate or a substrate made of different kinds of materials is used, the dispensed solvent may induce a coating unevenness.
To overcome the disadvantage, another spin coating method involves dispensing a coating liquid to a substrate while rotating the substrate at a first number of revolutions and then dispensing the coating liquid to the substrate while rotating the substrate at a second number of revolutions (see Japanese Patent No. 2638969, for example). That is, according to this method, dispensing of the coating liquid is divided into two or more steps in which the substrate is rotated at different numbers of revolutions.
This spin coating method cannot sufficiently spread the coating liquid over the entire substrate and can result in an incomplete coating because the first number of revolutions is low.
On the other hand, there is a spin coating method that involves dispensing a coating liquid to a substrate while rotating the substrate at a higher first number of revolutions and then dispensing the coating liquid while rotating the substrate at a lower second number of revolutions (see Japanese Patent Laid-Open Nos. 2000-157922, 2000-279874 and 2006-156565, for example).
According to the spin coating method described in the Japanese Patent Laid-Open No. 2000-279874, the coating liquid is spread over the entire surface of the substrate when the substrate is rotated at the first number of revolutions.
To the contrary, according to the spin coating methods described in the Japanese Patent Laid-Open Nos. 2000-157922 and 2006-156565, the number of revolutions is changed from the first number of revolutions to the second number of revolutions before the coating liquid is spread over the entire surface of the substrate. However, the amount of coating liquid dispensed when the substrate rotates at the first number of revolutions is enough to coat the entire substrate, and therefore, the coating liquid dispensed when the substrate rotates at the second number of revolutions is extra. In addition, the number of revolutions after dispensing (such as the number of revolutions for determining the film thickness) is set higher than the number of revolutions during dispensing, so that an excessive amount of coating liquid is discharged to the outside of the substrate when the substrate rotates at the higher number of revolutions.
In manufacture of a semiconductor device, a circular substrate is used. To the contrary, in manufacture of a photomask substrate, a liquid crystal display or the like, a rectangular substrate is used.
However, the spin coating method can be applied to any substrates. In particular, coating a stepped substrate requires a larger amount of coating liquid than coating a flat substrate.
In addition, as the diameter of the substrate or the size of the apparatus increases, the effect of the turbulence during rotation of the substrate increases, and there arises a possibility that the substrate flies when the substrate rotates at high speed.
As a result, the maximum value of the number of revolutions is lower than that for the old type apparatus, and the number of process restrictions increases.